this layer can reach the spectator at any given point； he hit upon a means of measurement that seemed to solve the hitherto inscrutable problem as to the atmospheric depth。 Like the measurements of Aristarchus and Eratosthenes； this calculation of Alhazen is simple enough in theory。 Its defect consists largely in the difficulty of fixing its terms with precision； combined with the further fact that the rays of the sun； in taking the slanting course through the earth's atmosphere； are really deflected from a straight line in virtue of the constantly increasing density of the air near the earth's surface。 Alhazen must have been aware of this latter fact； since it was known to the later Alexandrian astronomers； but he takes no account of it in the present measurement。 The diagram will make the method of Alhazen clear。 His important premises are two： first； the well…recognized fact that； when light is reflected from any surface； the angle of incidence is equal to the angle of reflection； and； second； the much more doubtful observation that twilight continues until such time as the sun； according to a simple calculation； is nineteen degrees below the horizon。 Referring to the diagram； let the inner circle represent the earth's surface； the outer circle the limits of the atmosphere； C being the earth's centre； and RR radii of the earth。 Then the observer at the point A will continue to receive the reflected rays of the sun until that body reaches the point S； which is； according to the hypothesis； nineteen degrees below the horizon line of the observer at A。 This horizon line； being represented by AH； and the sun's ray by SM； the angle HMS is an angle of nineteen degrees。 The complementary angle SMA is； obviously； an angle of （180…19） one hundred and sixty…one degrees。 But since M is the reflecting surface and the angle of incidence equals the angle of reflection； the angle AMC is an angle of one…half of one hundred and sixty…one degrees； or eighty degrees and thirty minutes。 Now this angle AMC； being known； the right…angled triangle MAC is easily resolved； since the side AC of that triangle； being the radius of the earth； is a known dimension。 Resolution of this triangle gives us the length of the hypotenuse MC； and the difference between this and the radius （AC）； or CD； is obviously the height of the atmosphere （h）； which was the measurement desired。 According to the calculation of Alhazen； this h； or the height of the atmosphere； represents from twenty to thirty miles。 The modern computation extends this to about fifty miles。 But； considering the various ambiguities that necessarily attended the experiment； the result was a remarkably close approximation to the truth。 Turning from physics to chemistry； we find as perhaps the greatest Arabian name that of Geber； who taught in the College of Seville in the first half of the eighth century。 The most important researches of this really remarkable experimenter had to do with the acids。 The ancient world had had no knowledge of any acid more powerful than acetic。 Geber； however； vastly increased the possibilities of chemical experiment by the discovery of sulphuric； nitric； and nitromuriatic acids。 He made use also of the processes of sublimation and filtration； and his works describe the water bath and the chemical oven。 Among the important chemicals which he first differentiated is oxide of mercury； and his studies of sulphur in its various compounds have peculiar interest。 In particular is this true of his observation that； tinder certain conditions of oxidation； the weight of a metal was lessened。 From the record of these studies in the fields of astronomy； physics； and chemistry； we turn to a somewhat extended survey of the Arabian advances in the field of medicine。
ARABIAN MEDICINE The influence of Arabian physicians rested chiefly upon their use of drugs rather than upon anatomical knowledge。 Like the mediaeval Christians； they looked with horror on dissection of the human body； yet there were always among them investigators who turned constantly to nature herself for hidden truths； and were ready to uphold the superiority of actual observation to mere reading。 Thus the physician Abd el…Letif； while in Egypt； made careful studies of a mound of bones containing more than twenty thousand skeletons。 While examining these bones he discovered that the lower jaw consists of a single bone； not of two； as had been taught by Galen。 He also discovered several other important mistakes in Galenic anatomy； and was so impressed with his discoveries that he contemplated writing a work on anatomy which should correct the great classical authority's mistakes。 It was the Arabs who invented the apothecary； and their pharmacopoeia； issued from the hospital at Gondisapor； and elaborated from time to time； formed the basis for Western pharmacopoeias。 Just how many drugs originated with them； and how many were borrowed from the Hindoos； Jews； Syrians； and Persians； cannot be determined。 It is certain； however； that through them various new and useful drugs； such as senna； aconite； rhubarb； camphor； and mercury； were handed down through the Middle Ages； and that they are responsible for the introduction of alcohol in the field of therapeutics。 In mediaeval Europe； Arabian science came to be regarded with superstitious awe； and the works of certain Arabian physicians were exalted to a position above all the ancient writers。 In modern times； however； there has been a reaction and a tendency to depreciation of their work。 By some they are held to be mere copyists or translators of Greek books； and in no sense original investigators in medicine。 Yet there can be little doubt that while the Arabians did copy and translate freely； they also originated and added considerably to medical knowledge。 It is certain that in the time when Christian monarchs in western Europe were paying little attention to science or education； the caliphs and vizirs were encouraging physicians and philosophers； building schools； and erecting libraries and hospitals。 They made at least a creditable effort to uphold and advance upon the scientific standards of an earlier age。 The first distinguished Arabian physician was Harets ben Kaladah； who received his education in the Nestonian school at Gondisapor； about the beginning of the seventh century。 Notwithstanding the fact that Harets was a Christian； he was chosen by Mohammed as his chief medical adviser； and recommended as such to his successor； the Caliph Abu Bekr。 Thus； at the very outset； the science of medicine was divorced from religion among the Arabians； for if the prophet himself could employ the services of an unbeliever； surely others might follow his example。 And that this example was followed is shown in the fact that many Christian physicians were raised to honorable positions by succeeding generations of Arabian monarchs。 This broad…minded view of medicine taken by the Arabs undoubtedly assisted as much as any one single factor in upbuilding the science； just as the narrow and superstitious view taken by Western nations helped to destroy it。 The education of the Arabians made it natural for them to associate medicine with the natural sciences； rather than with religion。 An Arabian savant was supposed to be equally well educated in philosophy； jurisprudence； theology； mathematics； and medicine； and to practise law； theology； and medicine with equal skill upon occasion。 It is easy to understand； therefore； why these religious fanatics were willing to employ unbelieving physicians； and their physicians themselves to turn to the scientific works of Hippocrates and Galen for medical instruction； rather than to religious works。 Even Mohammed himself professed some knowledge of medicine； and often relied upon this knowledge in treating ailments rather than upon prayers or incantations。 He is said； for example； to have recommended and applied the cautery in the case of a friend who； when suffering from angina； had sought his aid。 The list of eminent Arabian physicians is too long to be given here； but some of them are of such importance in their influence upon later medicine that they cannot be entirely ignored。 One of the first of these was Honain ben Isaac （809…873 A。D。）； a Christian Arab of Bagdad。 He made translations of the works of Hippocrates； and practised the art along the lines indicated by his teachings and those of Galen。 He is considered the greatest translator of the ninth century and one of the greatest philosophers of that period。 Another great Arabian physician； whose work was just beginning as Honain's was drawing to a close； was Rhazes （850…923 A。D。）； who during his life was no less noted as a philosopher and musician than as a physician。 He continued the work of Honain； and advanced therapeutics by introducing more extensive use of chemical remedies； such as mercurial ointments； sulphuric acid； and aqua vitae。 He is also credited with being the first physician to describe small…pox and measles accurately。 While Rhazes was still alive another Arabian； Haly Abbas （died about 994）； was writing his famous encyclopaedia of medicine； called The Royal Book。 But the names of all these great physicians have been considerably obscured by the reputation of Avicenna （980…1037